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If you know for sure the thermostat is working, it could be low on coolant and has air pockets in the system. Only one radiator hose should be warm until the engine reaches normal temp, and both heater hoses should be very warm.

Air in cooling system? Blockage? Thermostat failing? Collapsed hoses. there are many reasons why this problem exists. Hot coolant passes through your heater core which is generally located on the firewall of the car. If hot coolant does not circulate into the heater core, it will not pick up the heat and your car heating system will blow cool air. Coolant circulation is most likely the underlying issue. Your water pump may also be failing.

This is a common complaint for the 4.3 v-6 by GM, what happens over time is air gets into the cooling system, usually a small coolant leak at the intake manifold gasket. Ait gets in the cooling system and starts electrolisis. From this the coolant starts to solidify. If I'm correct your heater core is plugged. Remove the radiator cap (engine cold only) and inspect the coolant. The proper color should be red. If the elctolisis is taking place the coolant will turn to rust and look like mudd. If you find mudd in the radiator, most likely the heater core is plugged. This is a very expensive repair. You'll need to remove the raditor & sent it out to be boiled out. Replace the heater core, thermostat and the coolant hoses. You'll also need to perform a "prestone" flush. You may need to flush the cooling system monthly until the system is clean.